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+// Ceres Solver - A fast non-linear least squares minimizer
+// Copyright 2015 Google Inc. All rights reserved.
+// http://ceres-solver.org/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are met:
+//
+// * Redistributions of source code must retain the above copyright notice,
+//   this list of conditions and the following disclaimer.
+// * Redistributions in binary form must reproduce the above copyright notice,
+//   this list of conditions and the following disclaimer in the documentation
+//   and/or other materials provided with the distribution.
+// * Neither the name of Google Inc. nor the names of its contributors may be
+//   used to endorse or promote products derived from this software without
+//   specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+// AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+// ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
+// LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+// CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+// SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+// CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+// ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+// POSSIBILITY OF SUCH DAMAGE.
+//
+// Author: sameeragarwal@google.com (Sameer Agarwal)
+
+#ifndef CERES_INTERNAL_BLOCK_RANDOM_ACCESS_SPARSE_MATRIX_H_
+#define CERES_INTERNAL_BLOCK_RANDOM_ACCESS_SPARSE_MATRIX_H_
+
+#include <set>
+#include <vector>
+#include <utility>
+#include "ceres/mutex.h"
+#include "ceres/block_random_access_matrix.h"
+#include "ceres/collections_port.h"
+#include "ceres/triplet_sparse_matrix.h"
+#include "ceres/integral_types.h"
+#include "ceres/internal/macros.h"
+#include "ceres/internal/port.h"
+#include "ceres/internal/scoped_ptr.h"
+#include "ceres/types.h"
+#include "ceres/small_blas.h"
+
+namespace ceres {
+namespace internal {
+
+// A thread safe square block sparse implementation of
+// BlockRandomAccessMatrix. Internally a TripletSparseMatrix is used
+// for doing the actual storage. This class augments this matrix with
+// an unordered_map that allows random read/write access.
+class BlockRandomAccessSparseMatrix : public BlockRandomAccessMatrix {
+ public:
+  // blocks is an array of block sizes. block_pairs is a set of
+  // <row_block_id, col_block_id> pairs to identify the non-zero cells
+  // of this matrix.
+  BlockRandomAccessSparseMatrix(
+      const std::vector<int>& blocks,
+      const std::set<std::pair<int, int> >& block_pairs);
+
+  // The destructor is not thread safe. It assumes that no one is
+  // modifying any cells when the matrix is being destroyed.
+  virtual ~BlockRandomAccessSparseMatrix();
+
+  // BlockRandomAccessMatrix Interface.
+  virtual CellInfo* GetCell(int row_block_id,
+                            int col_block_id,
+                            int* row,
+                            int* col,
+                            int* row_stride,
+                            int* col_stride);
+
+  // This is not a thread safe method, it assumes that no cell is
+  // locked.
+  virtual void SetZero();
+
+  // Assume that the matrix is symmetric and only one half of the
+  // matrix is stored.
+  //
+  // y += S * x
+  void SymmetricRightMultiply(const double* x, double* y) const;
+
+  // Since the matrix is square, num_rows() == num_cols().
+  virtual int num_rows() const { return tsm_->num_rows(); }
+  virtual int num_cols() const { return tsm_->num_cols(); }
+
+  // Access to the underlying matrix object.
+  const TripletSparseMatrix* matrix() const { return tsm_.get(); }
+  TripletSparseMatrix* mutable_matrix() { return tsm_.get(); }
+
+ private:
+  int64 IntPairToLong(int row, int col) const {
+    return row * kMaxRowBlocks + col;
+  }
+
+  void LongToIntPair(int64 index, int* row, int* col) const {
+    *row = index / kMaxRowBlocks;
+    *col = index % kMaxRowBlocks;
+  }
+
+  const int64 kMaxRowBlocks;
+
+  // row/column block sizes.
+  const std::vector<int> blocks_;
+  std::vector<int> block_positions_;
+
+  // A mapping from <row_block_id, col_block_id> to the position in
+  // the values array of tsm_ where the block is stored.
+  typedef HashMap<long int, CellInfo* > LayoutType;
+  LayoutType layout_;
+
+  // In order traversal of contents of the matrix. This allows us to
+  // implement a matrix-vector which is 20% faster than using the
+  // iterator in the Layout object instead.
+  std::vector<std::pair<std::pair<int, int>, double*> > cell_values_;
+  // The underlying matrix object which actually stores the cells.
+  scoped_ptr<TripletSparseMatrix> tsm_;
+
+  friend class BlockRandomAccessSparseMatrixTest;
+  CERES_DISALLOW_COPY_AND_ASSIGN(BlockRandomAccessSparseMatrix);
+};
+
+}  // namespace internal
+}  // namespace ceres
+
+#endif  // CERES_INTERNAL_BLOCK_RANDOM_ACCESS_SPARSE_MATRIX_H_